This invention relates to the field of electromechanical micromachined structures and, more particularly, to micromachined magnetometers.
It is desirable for many purposes to be able to measure accurately a magnetic field. Magnetometers can be used as compasses and are useful in many areas, including virtual reality equipment, electronic games, and automotive products.
Conventional methods to sense magnetic fields include devices that measure the Hall effect, flux change, magnetoresistance, or giant magnetoresistance. While such methods can be effective, there are many areas in which these conventional devices could be improved. For example, it would be advantageous to improve the temperature coefficient and the stability of offsets in such devices, to obtain extremely high sensitivity with high dynamic range, to obtain true tilt sensing instead of total field sensing, to obtain higher linearity, to provide on-chip signal processing with an analog or digital output, to be able to measure magnetic fields in multiple axes with a single device, and to reduce the size of such devices. Also, it would be advantageous to be able to integrate a magnetometer with micromachined inertial sensors, such as accelerometers or yaw rate sensors, on a single chip.
The present invention is directed to a micromachined structure used as a magnetomneter. Applying a magnetic field B to a material with a magnetic moment m causes the material to experience a torque based on the cross-product of the magnetic field and the magnetic moment. In other words, the material experiences a torque L=mxc3x97B.
Micromachined silicon structures frequently are used to detect and measure acceleration through the use of differential capacitors. In such sensors, a movable mass is positioned between two plates so that one capacitor is formed by one plate and the mass and a second capacitor is formed by a second plate and the mass. An accelerometer based on this principle and a process for fabricating such an accelerometer are described in commonly assigned U.S. Pat. Nos. 5,345,824, 5,326,726, and 5,314,572, which are incorporated herein by reference.
According to the present invention, a single axis magnetometer is obtained by adding a ferromagnetic material to a micromechanical structure designed to detect rotation. In a preferred embodiment, a single device is used to measure magnetic fields in more than one axis. Devices that are sensitive to magnetic fields in three perpendicular axes can be placed on a single chip, along with accelerometers or other micromachined inertial sensors and resolving circuitry.
In a preferred embodiment, the micromechanical structure utilizes differential capacitors arranged so that rotation of the structure causes the center electrode of each differential capacitor to move closer to one of two fixed electrodes of the differential capacitor and further from the other fixed electrode. The resulting change in differential capacitance is proportional to the amount of movement, which, in turn, is proportional to the angular acceleration (or torque) applied to the structure.
A ferromagnetic material is deposited onto the movable center electrode of the micromechanical structure to give the structure a magnetic moment m. Preferably, the structure has a low moment of inertia, to minimize the sensitivity of the structure to any external mechanical angular vibration that would tend to cause the structure to move. With typical micromachined structures, the moment of inertia of the center electrode will be sufficiently low that in normal applications mechanical vibration will not cause sufficient movement to interfere with the functionality of the magnetometer.
A structure that rotates about an axis perpendicular to the plane of the substrate (i.e., that rotates about the Z-axis) is sensitive to a magnetic field along the X-axis if the ferromagnetic material has its magnetic moment aligned with the Y-axis. Similarly, such a structure is sensitive to a magnetic field along the Y-axis if the ferromagnetic material has its magnetic moment aligned with the X-axis.
Structures with perpendicular magnetic moments (e.g., aligned with the X and Y axes) can be obtained through various techniques. In one technique, the ferromagnetic material is applied in long, thin stripes on each of the two structures. The stripes are oriented along the X-axis on the first structure and along the Y-axis on the second structure. A magnetic field is applied to the device at 45 degrees from the X-axis. This causes the magnetic dipoles to be formed along the X-axis for the first structure and along the Y-axis for the second structure.
In a second technique, a hard ferromagnetic material is deposited on one structure and a softer ferromagnetic material is deposited on the other structure. A magnetic field is applied to the device in one direction, magnetizing both ferromagnetic materials. Then, a lower magnetic field is applied in a second direction, orthogonal to the first. The lower magnetic field is sufficient to re-magnetize the softer ferromagnetic material in the second direction, without affecting the magnetization of the harder ferromagnetic material.
A structure that is sensitive to a magnetic field along the Z-axis is formed from a plate suspended above the substrate so that it can rotate in either the X-axis or the Y-axis. A differential capacitor is formed from fixed plates on either side of the suspended plate. A ferromagnetic material is applied to the suspended plate with a magnetic moment oriented about an axis perpendicular to the axis about which the suspended plate can rotate.
Using one of these plate structures and one of the structures that rotates about the taxis permits the design of a dual-axis magnetometer in which the magnetic moments of both structures are oriented along the same axis.